This invention relates in general to color cathode ray tubes, and in particular to a color selection electrode assembly for use therein. Of equal significance, the invention is concerned with methods of manufacturing the electrode assembly as well as a cathode ray tube utilizing the assembly.
In general, a color selection electrode or "shadow mask" is a device which is disposed adjacent the luminescent phosphor screen that forms the target electrode of a color cathode ray tube, to control the landing pattern of one or more electron beams as they are swept across the screen. The shadow mask achieves color selection by partially shadowing the surface of the screen from scanning electron beams, permitting access to selected elemental phosphor areas by those beams. The choice of a color selection electrode for use in color television cathode ray tubes is, by and large, a choice between a non-tensed electrode and a tensed electrode. The most common type of color selection electrode used in color television receivers today is the non-tensed type.
In color picture tubes utilizing an untensed shadow m ask, there is a tendency on the part of the mask to "dome" (localized buckling) in those areas where a scene characterized by very high brightness is depicted. For example, in a scene where a high concentration of white is presented for an extended period of time, when the beams sweep that area of the screen the current in each beam peaks precipitously with an attendant localized heating of the mask. As a result of such a concentration of heat, that area of the mask expands and displaces itself from its original "cold" position to a position in which it does not effect proper masking of the writing electron beams. As a result, color purity is degraded. Moreover, because of its vulnerability to "doming", an untensed mask cannot accommodate the power density that a "doming-resistant" tensed mask can.
The general practice in cathode ray tubes manufactured for use in color television receivers is to position the untensed mask at an assigned location, relative to the phosphor screen, by suspending it from three preselected points disposed about the periphery of the tube's face panel. This suspension accommodates overall thermal expansion of the mask by causing the mask to be displaced toward the screen from its original position by provision of bi-metallic support springs; however, such provision can not resolve the above-described localized "doming" problem caused by concentrated heating in localized areas of the mask.
Insofar as the use of a tensioned color selection electrode is concerned, probably the most common use of such an electrode has been in connection with the cylindrical faceplate CRT produced by one color television manufacturer. In that tube, the color selection electrode comprises a grid formed of a multitude of parallel conductors tensed across a rigid frame. This grid serves to mask the writing beams to fall upon the desired light emitting phosphor.
The mask supporting frame is mechanically stressed, as by compressing it, prior to attaching the shadow mask thereto. Upon release of the compression force, restoration forces in the frame establish tension in the mask.
An advantage of utilizing a tensed mask resides in the fact that the mask, while under tension, will not readily submit to "doming". The mask retains its desired configuration until the heat generated by the scanning beams impinging thereon is sufficient to cause the area of the mask under bombardment to "relax" enough to negate the pre-established expansion of the mask.
Under extreme tube operating conditions, electron bombardment of a tensed mask can cause a series of grids of the mask to relax and cause color impurities. A cathode ray tube utilizing a tensed mask of the type adverted to above, the Sony Trinitron, is described in U.S. Pat. No. 3,638,063.
The color television cathode ray tube in most common usage today employs a faceplate which approximates a section of a large radius sphere. The shadow mask in such a tube, of course, is contoured to match the faceplate. A trend today is toward a flatter faceplate which, in turn, calls for a flatter shadow mask. One approach currently being pursued resorts to an untensed flat metal mask employed in conjunction with a substantially flat faceplate. However, a flat mask is inherently less mechanically stable than a curved mask. Accordingly, to acquire stability, resort is had to a thicker mask, for example, one having a thickness in the order of 10 to 12 mils. This is approximately twice the thickness of a conventional curved mask. However, when one goes to a flat 10 to 12 mil mask the aperture etching process is presented with horrendous problems. Specifically, in order to prevent aperture limiting of the beam at the outer reaches of the mask, as would be encountered in a 90 degree tube, the apertures have to be etched at an angle to the plane of the mask, rather then etched substantially perpendicular to that plane as is the case for a conventional curved mask. By way of resolving this aperture etching problem, applicant will teach the use of a color selection electrode assembly characterized by a thin, flat, tensed foil which, by virtue of its mounting, is mechanically stable and which is thin enough as to not be afflicted with the aggravated aperture etching problems posed by a thick mask.
Discussion of the Prior Art
An early example of a tensed shadow mask for use in a color television cathode ray tube is described in U.S. Pat. No. 2,625,734. The tensed mask described therein was created by resort to a process called "hot-blocking". The practice was to insert a flat mask between a pair of frames which loosely received the mask. A series of tapped screws joining the two frames served to captivate the mask then the screws were subsequently drawn-down. The loosely assembled frame and mask was then subjected to a heat cycle by positioning heated platens adjacent the mask to heat and thereby expand it. The frame, however, was kept at room temperature. When the mask attained a desired expansion, the frame screws were tightened to captivate the mask in its expanded state. The heating platens were then removed. Upon cooling down to room temperature, the mask was maintained under tension by the frame. The resultant assembly was then mounted inside the tube adjacent the phosphor screen.
U.S. Pat. No. 3,284,655--Oess is concerned with a direct viewing storage cathode ray tube employing a mesh storage target which is supported in a plane perpendicular to the axis of the tube. The mesh target comprises a storage surface capable of retaining a charge pattern which, in turn, control the passage therethrough of a stream of electrons. From a structural standpoint, it is proposed that mesh storage screen be affixed (no details given) to a circumferential ring that is disposed across the open end of envelope member One end of the ring is in contact with the edge of the envelope member which has a coating of glass frit applied thereon. The end wall of another envelope member, also coated with frit, is placed in contact with the other side of the ring so that the end walls of the envelope members now abut both sides of the ring. Thereafter this assembly is frit sealed to secure the ring and mesh target within the tube.
It is of particular significance that the electrode spanning the inside of the tube envelope is a mesh screen that is not said to be subject to tension forces. Moreover, the mesh screen is not a color selection electrode that serves to direct a writing beam to selected elemental areas of color phosphors. Finally, there is no criticality, perceived or discussed, as respects mesh target registration with the phosphor layer on the faceplate.
U.S. Pat. No. 2,813,213 describes a cathode ray tube which employs a switching grid mounted adjacent the phosphor screen to provide a post deflection beam deflecting force. Basically, it is proposed to employ a taut wire grid that is sealed in the tube envelope wall and which, in one embodiment, proposes the use of an external frame to relieve the tension forces applied by the taut grid to the glass wall "of the tube. In another embodiment, which is not pictorally disclosed but simply textually referred to, an arrangement is proposed comprising a glass donut-shaped structure into which the grid wires are sealed. This donut assembly is then inserted between the faceplate of the tube and its conical section. Thereafter, the patent notes, after the tube is assembled, the phosphors may be deposited on the faceplate by conventional photographic processes. The application of elemental color phosphor areas to the faceplate of a tube is, in itself, a formidable task; how this could be achieved with a grid structure in situ across the faceplate is dismissed in rather cavalier fashion. As will be developed herein, the subject invention teaches, inter alia, how an initially untensed shadow mask can be utilized to screen color phosphors on the faceplate of a color television tube.
Other examples of the prior art practice of utilizing a tensioned grid-type structure in a cathode ray tube environment are described in the following U.S. Pat. Nos.: 2,842,696, 2,905,845, 3,489,966, and 3,719,848.
Finally, and by way of emphasizing the extent to which the invention to be described departs from the prior art, attention is directed to U.S. Pat. No. 3,898,508 which shows and describes a faceplate and shadow mask (untensed) assembly representative of current practice.
Objects of the Invention
Accordingly, it is a general object of the invention to provide an improved color selection electrode arrangement for use in a color cathode ray tube.
It is another general object of the invention to provide a method of making an improved color selection electrode.
It is a further object of the invention to provide a color television picture tube which, in utilizing the improved color selection electrode arrangement, offers significant economic advantages over prior art tubes.
It is also an object of the invention to provide a method of manufacturing a color television cathode ray tube which, in utilizing the improved color selection electrode, effects substantial economies over prior practice.
It is an object of the invention to provide a color selection electrode of the tensed type which has the anti-doming attribute of tension-type electrodes, but without the power handling limitations of prior art tension electrode systems.
It is still another object of the invention to provide an envelope-captivated tensed color selection electrode system having the advantages of such systems, yet which is readily adapted to conventional color tube photoscreening methods and apparatus.